1. Field of the Invention
The present invention relates to a combination faucet, and more specifically to a combination faucet for mixing hot water with cold water by pressing a movable valve cylinder with a spring, which is composed of a material having a spring constant that varies with the temperature.
2. Description of the Related Art
An automatic temperature controlling combination faucet previously proposed to steadily control the temperature of water mixture of hot water and cold water by pressing a movable valve disc for controlling a mixing ratio of hot water to cold water with a shape memory alloy, which has a shape varied with the temperature (JAPANESE PATENT PUBLICATION GAZETTE No. 61-44062). As generally known, even if a physical force is applied to change the shape of a shape memory alloy having an original shape set at a specific temperature, the shape memory alloy recovers its original shape any time when it is exposed to the specific temperature. The shape memory alloy has a smaller heat capacity and greater sensitivity to the temperature variation than conventional temperature-sensitive elements, such as a thermo-wax.
In a known combination faucet a coil-formed shape memory alloy presses one end of the movable valve disc coil and a coil spring presses the other end of the disc. The coil-formed shape memory alloy is arranged to be in direct contact with the mixture of hot water and cold water, and has a given coil length at a specific temperature. The coil-formed shape memory alloy works in the following manner in response to a variation in the temperature of the water mixture.
When the temperature of the water mixture is stable at a preset temperature, the movable valve disc stops at a position of balance between the coil-formed shape memory alloy and the coil spring. When the temperature of the water mixture is varied to the specific temperature due to some outside disturbance, the coil-formed shape memory alloy generates a shape restoring force to recover the given coil length set at the specific temperature. The shape restoring force destroys the balance between the coil-formed shape memory alloy and the coil spring and moves the movable valve disc towards the coil spring or the coil-formed shape memory alloy. When the coil-formed shape memory alloy is set to change the coil length successively in a temperature range around the preset temperature, the coil-formed shape memory alloy varies its coil length in response to a change in temperature of the water mixture in the vicinity of the preset temperature so as to generate a shape restoring force continuously. This shifts the movable valve disc corresponding to the temperature change of the water mixture and varies the ratio of hot water to cold water, thereby keeping the temperature of the water mixture substantially equal to the preset temperature.
In the known combination faucet with a conventional shape memory alloy, the preset temperature of the water mixture is changed by manually applying a pre-load to the spring for pressing the movable valve disc. This structure does not allow the preset temperature to be changed from outside, for example, using a remote control. When the shape restoring force of the shape memory alloy balances with the resilient force of the coil spring at a certain temperature deviated from the desirable preset temperature, the temperature control only by the shape memory alloy cannot compensate sufficiently for a deviation of the actual temperature of the water mixture from a desirable preset temperature.
Another combination faucet previously proposed does not use a shape memory alloy, but controls the temperature of the water mixture by pressing a movable valve disc for controlling a mixing ratio of hot water to cold water with a temperature-sensitive element such as a thermo-wax and cancels a steady temperature deviation of the actual temperature of the water mixture from a target temperature by utilizing feed-back control (JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-31784). Such a steady temperature deviation results from temperature control of the thermo-wax in a temperature range that deviates from the target temperature, a time-based change in the volume variation of the thermo-wax, and a time-based change of each element where a load is applied due to the volume variation in the thermo-wax. In this combination faucet, a motor-induced driving speed of the movable valve disc is set at less than a thermowax-induced driving speed of the movable valve disc in order to prevent the feed-back control from causing undesirable hunting.
When the feed-back control executed in the combination faucet with a thermo-wax is applied to another combination faucet with a shape memory alloy, a difference in the temperature characteristics between the shape memory alloy and the thermo-wax may cause undesirable hunting. A change in the composition of a shape memory alloy by as little as 0.1% varies a temperature of Martensitic transformation representing the shape memory effects by as much as 10.degree. C. The difference in the Martensitic transformation temperature between different alloy ingots or even different lots may result in insufficient temperature control.
The feed-back control described above may not control the temperature of the water mixture sufficiently close to a target temperature in response to a season-oriented variation in the water supply temperature or pressure or a time-based variation in the spring constant of the shape memory alloy.
At a start of operation, the combination faucet spouts dead water retained in a supply conduit from a water heater to the faucet. During the dead water spouting, water mixture of a desirable temperature does not run from the combination faucet. The feed-back control or another temperature control executed during the dead water spouting makes the opening of the hot water supply unsuitably large, which may cause hot water to abruptly spout immediately after completion of the dead water spouting. Devices proposed for preventing such danger include a mixing control device for forcibly prohibiting operation of a mixing valve until a hot water supply temperature reaches a predetermined value (JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-125532) and another mixing control device for forcibly prohibiting operation of a mixing valve in a predetermined time period since a start of operation (JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-125533).
In the former method, however, the conventional combination faucet requires a temperature sensor in a hot water supply conduit for forcibly prohibiting operation of the mixing valve until a hot water supply temperature reaches a predetermined value. In the latter method, on the other hand, a non-operation time for prohibiting operation of the mixing valve should be set sufficiently long since a time required for spouting dead water varies with the mixing ratio of hot water to cold water and the flow of the water mixture. This undesirably retards the control to the target temperature.